1. 1.
Wireless Technologies
– A case study for IEEE 802.11g
OFDM system
Arpan Pal
Center of Excellence for Embedded Systems
TCS, Kolkata
arpan_pal@tcscal.co.in

2. 2.
Introduction
Cellular Wireless Systems & Wireless Networks
802.11g OFDM PHY development
System Description
Development Framework
Simulation Issues
Receiver Algorithms
Implementation Issues
Simulation Results
Security Algorithms
Areas of future - Convergence to 4G
Agenda

3. 3.
Wireless is the next giant leap in information services. The new paradigm
for connectivity enables business to operate
• faster
• better
• more cost effectively
• and more profitably
through the use of
• always on,
• always connected, and
• always available content and applications.
With the tremendous increase in wireless LANs, Mobile phones, PDAs,
and other mobile devices, the merging of computation and
telecommunication technologies is a fundamental part of modern society.
Can broadly be classified into two types – Cellular Wireless Systems
and Wireless Networks
Introduction

4. 4.
CellularWireless Systems Roadmap
9.6 k
64 k
384 k
1000 k
2000 k
5000 k
1995 2000 2005
AMPS
TACS NMT
IS136
GSM
IS95A
IS95B
GSM-GPRS
1980
CDMA2000
W-CDMA
4G3G2.5G2G1G
???

5. 5.
Wireless Networks PAN – IEEE 802.15
LAN – IEEE 802.11
MAN – IEEE 802.16

6. 6.
OFDM Overview
• Multi-Carrier Modulation Technique
• Carrier spacing kept minimum maintaining orthogonality
• Multi-path robustness due to multi-carrier
• Reduction of ISI through Guard Bands
• Robust against narrow-band interference
• Efficient FFT based receiver structures
• Simpler Frequency Domain Channel Equalization
• Simpler Receiver Synchronization Techniques
• High peak-to-average power levels
• Susceptible to RF Front-end non-linearity
• Susceptible to LO frequency offset / drift

7. 7.
802.11g OFDMPHY
Assemble
frame
Scrambler
Convolution
Block
Interleaver
Bit Mapper IFFT Add Guard
Interval
Window
MAC Layer
DAC
RF Transmitter
Transmit
Remove
Guard
Interval
FFT Channel / Phase
Correction
De-mapper
De-interleaver Viterbi
Decoder Descrambler Disassemble
Frame
Channel Estimator
Frequency/Phase Correction
MAC Layer
ADC
RF Receiver
AGC
AFC
Receive
Management Entity
PLCP
PMD
PMD
PLCP
Frame
Sync &
Coarse
Frequ.
Correct
Receiver Sync
PLME
Preamble & Pilot Insertion

8. 8.
Development Framework
Simulation Environment
Fixed point Model
Floating point Model
MATLAB model
Analysi
s
Result
C
System level model
Analysi
s
Result
VHDL / Verilog
Simulation
Synthesis
Rest of the process

9. 9.
Simulation Issues
• Proper selection of channel models with various delay-spreads and Doppler
shifts.
• Proper simulation of Sampling Clock error.
• Proper simulations of Phase and Frequency Error.
• Proper modeling of Phase Noise.
• Proper modeling of I-Q imbalance.
• AGC
• Proper modeling of the LNA and anti-aliasing filter.
• AWGN noise.

10. 10.
ReceiverAlgorithms
• Time Synchronization
Packet Detection
Energy Based
Frame Synchronization
Short Training Sequence Cross-Correlation Based
False Alarm Reduction
Short Training Sequence Auto-Correlation Based

11. 11.
ReceiverAlgorithms
• Frequency Synchronization
Frequency Offset Estimation
Coarse estimate based on Short Training Sequence Cross-Correlation
Fine estimate based on Long Training Sequence Cross-Correlation
Frequency Offset Correction
Time domain (pre-FFT) rotation based on estimated offset
Carrier Phase Tracking
Phase offset due to residual frequency offset and sampling clock error
Pilot based estimation for phase
Each OFDM symbol contains 4 pilots
Frequency domain (post FFT) rotation for phase correction

12. 12.
ReceiverAlgorithms
• Channel Estimation
Assumes quasi-stationary channels (does not change within a packet)
Channel Transfer function estimated from long training sequence (LTS)
Estimated Channel Transfer Function
= FFT(Received LTS) / FFT(ideal LTS)
Takes care of indoor channels along with gain variation
Channel compensation done post FFT by dividing with estimated
Channel Transfer Function

13. 13.
Implementation Issues
• Use of Radix-22
FFT /IFFT algorithm instead of a Radix-2 or a Radix-4
implementation.
• Use of CORDIC (COordinate Rotation DIgital Calculation) for performing
complex multiplication and division.
• Determining the scaling factor to be used after every stage of the FFT block.
• Deciding on the number of iterations to be used for implementing CORDIC.
• Approximating all sqrt(x2
+ y2
) with (|x| + |y|) for hardware simplification
and altering the various threshold values accordingly.
• LUT implementation of various mathematical calculations.
• Deciding on the number of bits to be used for ADC
• Fixing the number of bits to be used for implementing FFT/IFFT.

14. 14.
Simulation Results
2
3
4
5

15. 15.
Simulation Results
Channel Model 3 used

16. 16.
Security Algorithms
• “An Alternative Approach for Enhancing Security of Wireless Networks using
Physical Layer Encryption” – patent filed
• Provides enhanced security against
- Data Privacy
- Data Forgery
- Denial of Service
• Two KEY Security method (secure KEY delivery assumed)
• KEYS used to encrypt/modify physical layer parameters like
- Error Control Coding Rate
- Type of Modulation / Constellation Mapping
- Length of Packet
- Interleaving Pattern
- Phase offset
• Contribution presented in IEEE 802.20 Standard body meeting in September,
2003

17. 17.
Areas of future – Convergence to 4G
• 4G is whatever that is beyond 3G
• To be used for Real-Time Video Delivery and similar applications
• QOS is important
• Calls for more efficient Modulation, ECC and Equalization
• Incorporation of Mobility and stringent Multipath including non- LOS
scenario into Wireless Networks can lead to 4G
• OFDM and UWB likely candidates for 4G PHY
• Space-Time Diversity, Smart Antenna Processing and Multiple-Input-
Multiple-Output (MIMO) systems are also likely to be used
• CDMA with its multi-user capabilities can provide the access mechanism
• Calls for implementation of a MAC layer that seamlessly integrates all the
above features of PHY